This invention relates to a pump; more particularly to a liquid helium pump having a superconducting electromagnetic drive which includes two stationary superconducting coils connected to the pump housing and a superconducting coil which is movable by means of the field generated by the stationary coils and which is fixedly attached to the pumping member proper of the pump.
Pumps of the above-outlined type are needed for driving liquid or supercritical helium in a closed circuit or, in general, for displacing liquid helium.
Particularly in the field of fusion technology, the use and operation of coreless fusion magnets are indispensable. The coils of the fusion magnets--which generate magnetic fields of a flux density in the order of a few Tesla--are frequently constituted by hollow superconductors, which may have a length up to 1,000 m and which are to be cooled with single phase supercritical helium. The displacement of the helium is effected at a pressure above 2.4 bar and at a temperature of approximately 4.2 K. The pressure difference between inlet and outlet is in the order of magnitude of 1 to 4 bar.
In an article entitled "A Reciprocating Liquid Helium Pump Used for Forced Flow of Supercritical Helium" by G. Krafft et al. (Cryogenics, February 1978), there is disclosed a piston pump for driving liquid helium. The pump drive is arranged externally of the cryostat and therefore the piston rod has to pass through the cryostat wall. Such an arrangement requires a vacuumtight and heliumtight seal and is involved with substantial expense. These difficulties are circumvented in another known liquid helium pump described in an article entitled "Heat Transfer by the Circulation of Supercriticial Helium" by H. H. Kolm et al. (Advances in Cryogenic Engineering, Volume 11, Plenum Press, New York, 1965). In this arrangement, the ferromagnetic piston is, with bellows, accommodated in a pump housing and a solenoid for driving the piston is arranged externally of the pump housing. The pump housing which necessarily is made of a non-magnetic material, on the one hand, increases the gap between the solenoid and the piston and, on the other hand, it does not eliminate undesired effects of foreign (external) magnetic fields. It is another disadvantage of this type of pump that it has only a small output of approximately 6.4 cm.sup.3 /s.
In an article entitled "An Electrically Pumped Liquid Helium Transfer System" by B. Darrel et al. (Advances in Cryogenic Engineering, Volume 11, Plenum Press, New York, 1965), there is disclosed a liquid helium pump wherein the superconducting driving coil is a disc coil mounted at an end of a bellows-equipped superconducting niobium piston received in the pump housing. The piston is moved by the driving coil by attraction and repulsion. The forces exerted on the piston are limited by the relatively low critical field of niobium of about 0.6 Tesla. An operation of this type of pump in external magnetic fields above this field value is not possible. The output of this pump too, is low; it is only approximately 7 cm.sup.3 /s.